There are numerous television and video digital signal sampling rate standards. For example, a sample rate of 13.5 MHz is compatible with the 4:2:2 Component Digital Standard (CCIR 601). Sampling at this rate produces 720 active video samples per horizontal linearized line and 483 vertical lines on a standard television tube. This level of resolution is also referred to as standard definition television (SDTV). SDTV displays active video information 483 of 525 lines per image frame in interlaced mode. High Definition Television (HDTV) typically samples in the range of sample in the range 72 MHz to 81 MHz. The Society of Motion Picture and Television Engineers, SMPTE, has approved an HDTV format of 1920 samples (pixels) per horizontal line and 1035 vertical lines. In addition to this format other television formats exist, such as the D2-MAC, PAL and SECAM standards in Europe and the Wide Screen Television (WST) standard.
The horizontal and vertical pixel resolution may best be thought of as a normalized horizontal and vertical dimension for the resulting image. In this context, the images are normalized in terms of a standard sized pixel. Thus, different image resolution is translated directly to a different image size.
With the many standards in existence, it is desirable to be able to convert from one standard to another, such as for the display of a television/video signal recorded in one standard format on a television set designed for display of a different standard format. It is also desirable to keep any image distortion caused by the conversion to a minimum. Because the initial standard and final standard may not be known prior to conversion, it is also desirable to have a more universal system which can handle conversions among all of the formats.
In some instances, this spatial conversion is referred to as video resizing. For example, video resizing allows a full sized motion picture film (35 mm, 24 frame per second) having an aspect ratio greater than 16 by 9 to be displayed on an NTSC television set with an aspect ratio of 4 by 3 without having to "letter box" the output display. In the digital domain, video resizing requires that the input signal be digitally resampled.
Digital resampling produces a different representation of the digital input signal by calculating points of the displayed signal that did not necessarily exist in the original signal. New samples are generated at instants which did not previously have samples through an interpolation technique. A flexible digital interpolation filtering architecture is capable of resizing a video/television line to an arbitrary size. An example of video resizing is described in U.S. Pat. No. 08/317,474 issued Nov. 4, 1994, entitled FILTER SELECTION CIRCUIT FOR DIGITAL RESAMPLING SYSTEM, which is incorporated by reference herein.
Another example of video resizing takes place in picture-in-picture television displays. U.S. Pat. No. 4,652,908 issued Mar. 24, 1987 entitled FILTERING SYSTEM FOR PROCESSING A REDUCED RESOLUTION VIDEO IMAGE, describes a system for video signal processing to produce a reduced size image for display inside of a larger image television screen. This system involves a reduction of the signal to an already known, smaller picture size.
In order to carry out the resizing, it is necessary for both horizontal and vertical resampling to take place. For example, to convert a 1920.times.1035 HDTV signal to a 720.times.483 SDTV signal it is necessary to have both a horizontal and a vertical resampling operation. For a real time or near real time applications, it is desirable for the horizontal and vertical resizing operations to take place at substantially the same time. A block based approach for carrying out this process has been used, whereby each picture (i.e. image frame) is broken up into block-shaped pieces and each piece is resampled by a separate processor. Each of these multiple processors works in parallel on a single block. This process can be awkward in that it introduces problems relating to interpolation and overlap requirements. Once processing is complete for each of the separate subdivided blocks, the final image is created by piecing together the separately processed blocks.
Another method for carrying out both horizontal and vertical resampling is to use a Finite Impulse Response (FIR) filter, which includes a tapped delay line, for the vertical processing where the total delay provided by the delay line is on the order of several line intervals. This method, however, requires very high overhead because of memory requirements. If sufficient processing does not take place, (for instance because of insufficient memory) the range and quality of the conversion will be degraded.